Low-Cost Fabrication of Printed Electronics Devices through

Letter www.acsami.org

Low-Cost Fabrication of Printed Electronics Devices through Continuous Wave Laser-Induced Forward Transfer Pol Sopeña,†,‡ Javier Arrese,‡,§ Sergio González-Torres,†,‡ Juan Marcos Fernández-Pradas,†,‡ Albert Cirera,‡,§ and Pere Serra*,†,‡ †

Department of Applied Physics and §MIND, Engineering Department: Electronics, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain ‡ Institute of Nanoscience and Nanotechnology (IN2UB), Universitat de Barcelona, Joan XXIII S/N, 08028 Barcelona, Spain S Supporting Information *

ABSTRACT: Laser-induced forward transfer (LIFT) is a direct-writing technique that allows printing inks from a liquid film in a similar way to inkjet printing but with fewer limitations concerning ink viscosity and loading particle size. In this work, we prove that liquid inks can be printed through LIFT by using continuous wave (CW) instead of pulsed lasers, which allows a substantial reduction in the cost of the printing system. Through the fabrication of a functional circuit on both rigid and flexible substrates (plastic and paper), we provide a proof-of-concept that demonstrates the versatility of the technique for printed electronics applications. KEYWORDS: laser-induced forward transfer, laser printing, printed electronics, paper electronics, gas sensors

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bubble, expansion of which results in the development of a liquid jet that propagates away the film (Figure 1b). A receiving substrate placed in front of the ink free surface intercepts the jet and allows for collecting the ejected liquid, thus leading to the formation of the sessile droplet that ultimately becomes the printed pixel.10−13 Since the jet is generated from an unconstrained liquid, and not by means of flow through a nozzle, this sets fewer restrictions concerning the printable particle size.14 The transient character of the jetting dynamics observed in the LIFT of inks is inherent to the use of pulsed lasers, apparently a major requirement of the technique. Pulsed lasers, though, are expensive, or at least considerably more expensive than analogous continuous wave (CW) lasers with the same output power. This issue can overshadow the interesting attributes of LIFT, and thus hinder or delay the spread of the technique in the printed electronics industry. With the aim of overcoming the drawback, here we propose carrying out the LIFT of liquid inks with CW laser radiation; this unconventional approach should allow a substantial reduction in the cost of acquisition of the laser source which in turn should impact the final production costs of the printed devices. CW lasers have already been employed for the digital transfer of materials,15 as in the well-known laser-induced thermal imaging

nkjet printing is probably the most widespread technique for the digital manufacture of printed electronics devices.1−3 With a long history of development in the field of the graphic arts, inkjet printing was quickly adapted to print the new materials required by the new applications with considerable success.4 However, the technique presents constraints related with the rheological properties of the printing ink which can limit its application in some instances. Among them, the size of the particles loading the ink: a particle size on the order of 1/ 100th of the output nozzle diameter usually represents the upper threshold for a printable ink (due to nozzle clogging issues).5 Besides, the rheology of the ink becomes fundamental and an inverse of the Ohnesorge number Z laying between 1 and 14 is required to form stable drops.6 These issues can easily exclude several interesting materials from being printed through inkjet printing. Nanostructured materials like nanowires, nanofibers, or nanotubes, very promising in many electronic applications (smart systems, sensors, devices, LEDs, or batteries), can be hardly printed through inkjet printing without previously modifying the extreme aspect-ratios that provide them with their unique functional properties.5,7 Laser-induced forward transfer (LIFT) has been revealed as an interesting alternative to inkjet printing in that it allows printing inks of a wide range of materials with little limitations concerning their rheological properties. The principle of operation of LIFT is rather simple (Figure 1a): a pulsed laser beam is focused on a thin film of the ink to be printed through a transparent donor substrate.8,9 The absorption of the laser pulse at the film interface leads to the formation of a cavitation © 2017 American Chemical Society

Received: March 28, 2017 Accepted: August 23, 2017 Published: August 23, 2017 29412

DOI: 10.1021/acsami.7b04409 ACS Appl. Mater. Interfaces 2017, 9, 29412−29417

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ACS Applied Materials & Interfaces

with an average diameter below 60 nm) as donor material and glass as receiving substrate in a typical LIFT configuration with a gap between donor and receiving substrate of 150 μm (see the Supporting Information for details). With the aim of printing conductive lines we scanned the donor film with a CW-Nd:YAG laser beam (1064 nm wavelength, 100 μm beam waist diameter) following straight lines in the way presented in Figure 2a at the maximum scan speed available with our system (600 mm/s) and at different output powers. In spite of the fact that the wavelengths in the visible range would be better suited for transfer attending to the radiation absorption properties of the Ag ink,17 CW-LIFT is intended to be applied to a broad range of instances, many of which can include inks of materials with different optical properties. Therefore, the use of such an ubiquitous laser radiation as the Nd:YAG fundamental wavelength (1064 nm) seems a reasonable choice in a feasibility study like this, even if its wavelength is not the optimum one for that particular ink in terms of absorption (for a more detailed analysis of the inks absorption characteristics, please, see the Supporting Information). In fact, even in instances wherein the ink is completely transparent to the laser radiation, it is always possible to use an intermediate absorbing layer between the donor substrate and the donor liquid film, a strategy commonly employed in pulsed LIFT.10,18 Remarkably, stable continuous lines were obtained in most cases in the lines printing experiment (Figure 2b). The corresponding line widths, around 200 μm (the irregular contour represents a deviation of about 15% around the average for 1.0−4.0 W laser powers), are perfectly acceptable in many printing applications; in fact, most commercially available printed electronics devices, as well as most realizations displayed in the literature, have similar (and even larger) minimum feature sizes.19 Nevertheless, smaller features could be even achieved through tighter focusing of the laser beam or through improved radiation absorption in the liquid donor film (through the use of either a different laser wavelength or an intermediate absorbing layer). In spite of the presence of some spray around the edges, the lines appear to be uniformly covered with ink all along their lengths for most of the investigated laser powers; only at the highest powers (between 6 and 7 W) discontinuous deposit is observed in the center of the lines. Scanning electron microscopy (SEM) confirms that at low powers, the lines are indeed uniformly coated with Ag nanoparticles (Figure 2c). In fact, at the lowest powers the particle distribution reproduces well that of the donor film. The SEM images also reveal that as the laser power is increased above 2 W, the nanoparticles concentration decreases, and larger particles (up to 0.5 μm in diameter) are found randomly scattered onto the coating, sometimes forming aggregates. These aggregates could be attributed to nanoparticles melting and coalescence taking place during laser irradiation, being most prominent at the highest powers. Similar micrometersized ejecta have been observed during the ablation of metallic nanoparticle films with pulsed lasers.20 The thermodynamic size effect of nanoparticles coupled to the reduced thermal conductivity of nanostructured materials can result in nanoparticle melting at substantially lower temperatures than those required for bulk metal, which could account for the formation of the aggregates even at the relatively low laser powers of these experiments.21 The different hue and reflectivity of the borders of the line at 7.0 W (Figure 2b) are probably due to the much lower density of nanoparticles, as well as to the presence of the aggregates. Finally, it is also remarkable that in spite that very

Figure 1. (a) Sketch of the setup and principle of operation of pulsed LIFT. The laser beam is scanned along the donor film at a certain scan speed and repetition rate. Each laser pulse results in the formation of a long jet of ink, which contact with the receiving substrate leads to the printing of a droplet. (b) Stop action movie of a printing event. In all the frames, the donor film is located in the top of the image and the receiving substrate in the bottom, and the laser beam is impinging from above. The images were acquired with a time-resolved microscope in shadowgraphy configuration; the acquisition delay respect to the laser pulse is indicated above each frame, and the aperture time was always 100 ns. It is observed that the jetting dynamics characteristic of pulsed LIFT results in the formation of a printed droplet through contact of the jet with the receiving substrate.

(LITI) technique.16 However, in those instances, transfer was always carried out from solid donor films. The CW-LIFT approach with liquid donor films that we propose here should allow broadening considerably the range of printable materials.8 At the same time, in that transfer takes place in liquid state and with no phase change, the approach would be compatible with the use of inkjet and screen-printing inks, and therefore with all the already existing technologies in the printed electronics industry. Upon scanning a focused CW laser beam on a donor film of a commercial ink, we prove the feasibility of CW-LIFT by means of printing continuous and stable conductive lines which can be used as interconnects in printed electronic circuits. Fast photography of liquid ejection allows us to demonstrate that material transfer proceeds through a spray mechanism apparently unrelated with the jets generated during LIFT with pulsed lasers. Finally, as a proof-of-concept for the new approach we fabricate a circuit containing a gas and a temperature sensor wherein all the printed elements have been deposited through CW-LIFT. To this aim, we have used two inks of very different particle size each, and we have printed them on both rigid (glass) and flexible (polyimide and paper) substrates in order to show the versatility of the technique for printed electronics applications. In the first printing experiments, we used a commercial inkjet printing Ag ink (water-based suspension of Ag nanoparticles 29413


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Figure 2. (a) Sketch of the setup and principle of operation of CW-LIFT. The laser beam is scanned along the donor film at a certain scan speed, which results in the printing of a continuous line of ink on the receiving substrate. (b) Optical microscopy images of lines of Ag ink printed through CW-LIFT on glass at a scan speed of 600 mm/s and different laser powers; the laser power and corresponding intensity (power per unit area) are displayed besides each line. The images correspond to dried ink before sintering. It is observed that continuous lines free from bulging are obtained in all cases. (c) SEM images with details of the center of the printed lines. It is observed that the concentration of nanoparticles in the line decreases with laser power, whereas the presence of large particles and aggregates increases. (d) Plot of both thickness (■) and sheet resistance (○) versus laser power and intensity. The minimum sheet resistance is obtained at fairly low laser powers (1−2 W). (e) Stop action movies of liquid ejection during CW-LIFT acquired through fast photography at different laser powers and at a speed of 300 mm/s. In all the frames the donor film is located in the top of the image and there is no receiving substrate (though its eventual position is indicated in the top frames); the laser beam is impinging from above and scanning the donor film from right to left. The acquisition delay respect to an arbitrary time t is indicated above each frame, and the aperture time was always 5 μs. The blue arrow indicates the position of the ejected ink front. The full video can be seen in Movie S1. It is observed that liquid transfer in CW-LIFT proceeds through a spraying dynamics.

thickness achieved being slightly lower than that obtained with inkjet printing single scans, the corresponding sheet resistancethe parameter that ultimately determines the functionality of the printsis perfectly comparable to those characteristic of inkjet printing.25 These results, together with the possibility of printing long continuous lines demonstrated in the previous paragraph, conclusively prove that CW-LIFT is feasible for printing conductive lines from inks to be used as interconnects in electronic circuits. Yet there is another issue that deserves attention: the optimum lines were obtained at fairly low powers in these experiments (which could be substantially reduced through tighter focusing of the laser beam). This is truly remarkable in that it means that very inexpensive lasers can be used, which positively reverts on the cost reduction initially aimed through the approach of operating in the CW regime. In fact, robust and stable CW laser diodes with powers of about 1 W can be purchased for as little as a few hundred euros. Aside from cost reduction, operating with low power CW lasers presents additional advantages, like the possibility of using truly portable lasers that are easy to integrate in a production line, not to mention energy consumption and safety issues.26

long lines were printed (up to 6 cm), bulges were never found; this is noticeable, because bulging is one of the most problematic defects in printed interconnects in that they can lead to undesired short-circuits between adjacent lines.22 Tiny droplets arising from the spray can reach distances up to about 200 μm away from the border of the line; however, they do not contribute to the line electrical properties, and therefore they do not compromise its functionality, as will be shown later. The plot of the average thickness of the line versus laser power (Figure 2d) displays a maximum value of around 90 nm at powers between 1 and 2 W, followed by a monotonous decrease at increasing powers (the thickness values above 5.0 W are not very significant due to the depletion of nanoparticles in the center of the lines). An equivalent plot for the sheet resistance of the lines after sintering in an oven (see the Supporting Information for details on sintering conditions) shows the reversed behavior, with a minimum sheet resistance of about 0.4 Ω/□ at the same powers of 1−2 W, which corresponds to a minimum resistivity of about 3 μΩ cm (see the Supporting Information for details on the evolution of resistivity with laser power). These values are comparable to and even better thanthose usually obtained with laser sintered metallic nanoinks.23,24 In spite of the maximum 29414


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Figure 3. Gas and temperature sensor printed through CW-LIFT. (a) Ag ink electrodes printed on glass at a laser power of 1 W and a scan speed of 600 mm/s; the image corresponds to sintered electrodes. Time to print the entire set: 2 s. (b) Finished sensor displaying the gas sensing pad (CNFs printed through CW-LIFT at the same conditions onto the area delimited by the interdigitated electrodes), the surface mount thermistor soldered to the outer set of electrodes, and the contacts for measurements. (c) The same sensor printed on polyimide. (d) The same sensor printed on paper. e) SEM image of a detail of the gas sensing pad, which displays the relatively large CNFs on a background of Ag tiny nanoparticles. (f) Resistance versus temperature curve of the surface mount thermistor corresponding to the sensor printed on glass (■), polyimide (○), and paper (Δ). Dynamic resistance of the gas sensor to humidity for the device printed on (g) glass, (h) polyimide, (i) paper, and (j) to NO2 for the device printed on paper. These plots prove the functionality of the printed sensors.

lower than that, we can consistently assume the onset of pool boiling in the irradiated portion of the ink (the consistency of this hypothesis with the characteristic length and time scales of the process is discussed in detail in the Supporting Information). The splash originated by the bursting tiny bubbles during pool boiling would account for the observed atomization of the liquid, and therefore for the spraying dynamics. Furthermore, the images also reveal that the intensity of the spray peaks at 1−2 W to later decrease at higher powers, which correlates well with the line thickness evolution observed in Figure 2d. This can also be correlated with the dark band observed in the center of the printed lines at high powers (Figure 2b). In experiments of laser sintering of metallic nanoparticle films it is observed the displacement of ink toward the edges of the laser scanned lines due to thermocapillary flow induced by laser heating.17 That flow would be responsible for the depletion of material along the center of the irradiated lines, and it would therefore account for the decrease in the intensity of the spray observed above 1.0 W. A final consequence of this would be the absence of deposited material observed in the central part of the lines printed at the highest powers (Figure 2b). To provide a convincing proof-of-concept for the feasibility of the technique in printed electronics applications, we fabricated a functional device through the CW-LIFT of liquid inks: a gas and a temperature sensing circuit. Aside from the environmental interest of both magnitudes, temperature affects the response of most gas sensors, with much work devoted to this with the proposed gas sensing material.27 Furthermore, sensing is an important sector in the field of printed electronics, so that with our choice we are aiming at an application with broad scope and significant potential impact. In fact, this is one of the reasons why a considerable variety of sensors have also been fabricated through conventional pulsed-LIFT.28,29 We first printed the same Ag ink used in the previous experiments at a laser power of 1 W and a scan speed of 600 mm/s in order to

There is an important difference between CW and conventional pulsed LIFT yet to be considered, the transfer process. The jetting dynamics of pulsed LIFT (Figure 1b) is hardly compatible with the continuous nature of the new approach. So, how does liquid transfer proceed in CW-LIFT? In order to answer this question we carried out fast photography of ink ejection in the absence of any receiving substrate at different laser powers. A slower speed (300 mm/s) than that used in the printing experiments was set in order to acquire a number of frames high enough to properly reproduce the movie of the process with a magnification allowing to visualize the ejected liquid under all the analyzed conditions. The selection of frames presented in Figure 2e reveals that the laser beam atomizes the liquid as it scans the donor film and projects it toward the receiving substrate as a spray, a transfer dynamics completely different from that of pulsed LIFT (see Movie S1 for full videos). The broad spray angle does not represent a critical problem for printing since the typical gap between donor and receiver in LIFT is small enough to allow ink collection before there is too much spread on the receiving substrate. In fact, the observed dynamics allows understanding the tiny sprayed droplets around the edges of the printed lines described before. The unexpected spray dynamics could be attributed to the longer irradiation times compared to pulsed LIFT (in fact, the laser peak intensities during the CW-LIFT of liquids are more than 4 orders of magnitude smaller than those of conventional pulsed LIFT). In contrast with the pulsed case, wherein a single cavitation bubble generated at the solid−liquid interface was responsible for jetting, now practically the entire thickness of the irradiated film is heated by the laser beam, and at a much slower rate. We have estimated a heat penetration depth of around 30 μm and a total irradiation time of 340 μs for a scanning speed of 300 mm/s, an irradiation time several orders of magnitude above the few nanoseconds characteristic of pulsed LIFT. Since the estimated time required for boiling inception of the entire liquid film (tB = 8 μs) is substantially 29415


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observed in the sensor deposited on glass, in good agreement with the specifications of the device. A similar behavior is obtained with the sensor printed on polyimide, though with a higher contact resistance. Worse results are obtained in the case of paper, where heat transfer issues and chemical reactions could arise. Concerning the CNFs response to gases, it was first investigated before water vapor for the three different substrates with unheated CNFs (Figure 3g−i). In the case of glass, the CNFs resistance increases with the water pulses as expected, showing good stability within a sequence of pulses. However, the baseline increases along the different sequences of pulses. In the case of polyimide a higher resistance is observed, as well as a slightly longer response/recovery time, but improved baseline. Nevertheless, both responses are in good agreement with previous results.27,30 In the case of the sensor on paper, the hydrophilicity of this substrate seems to induce a backfilling process when recovering the sensor with synthetic air that results in rather inconsistent measurements. Paper does not seem, therefore, an adequate substrate for water vapor sensing. To test the sensor with a more suited gas, we chose NO2, which should be free from backfilling issues. In this case, the sensor response is similar to that previously obtained for that gas on a polyimide27 and on a ceramic substrate.30 It should be noted, nonetheless, that the sensing material is not heated in the present case. A simple heating at 40−60 °C would stabilize the baseline and improve the response/recovery times, eventually providing even better results. Alternatively, an advanced selfheating strategy of the CNFs at the nanoscale could be applied with the same aim.30 In conclusion, we have proved that CW-LIFT allows the digital printing of functional inks from liquid donor films at high speed and with few restrictions concerning the size of the particles loading the ink. As with conventional pulsed LIFT, in some instances this approach can present significant advantages over other printing techniques, like inkjet printing, in that it allows broadening the range of materials which can be digitally printed; nanostructured materials, for example, can be successfully transferred without altering the high aspect ratios that confer their unique functional properties. But it also represents a substantial advantage over pulsed LIFT itself, since the use of CW instead of pulsed lasers can result in a remarkable cost reduction in the setting up of the printing unit, even more considering that we have demonstrated that optimum results can be obtained with relatively low powers, and therefore with very inexpensive lasers. Finally, through the fabrication of an operative gas and temperature sensing circuit, we have provided a valuable proof-of-concept of CW-LIFT in at least such a relevant printed electronics application as sensors manufacturing, and have shown that the technology is compatible with the use of flexible plastic and paper substrates.

produce a series of narrow interdigitated electrodes, as well as additional wider interconnects, these last ones generated by printing parallel overlapping lines (Figure 3a). The electrical characterization of the printed elements showed that the interdigitated electrodes are in open circuit condition, which proves that the residual spray around the edges of the printed lines does not compromise the functionality of the electrodes. We then printed in identical irradiation conditions a square pad of carbon nanofibers (CNF), the gas sensing material, from an aqueous suspension onto the interdigitated electrodes, and assembled a surface mount thermistor (the temperature sensing element) to the printed interconnects alongside four contacts for measurements (Figure 3b). CNFs are a promising material in gas sensing applications thanks to their high sensitivity27 as well as to their elongated graphene-like structure, which allows devising strategies for self-heating in the nanoscale. On the other hand, the inclusion of the thermistor allows proving the compatibility of the stripes printed through CW-LIFT with surface-mount technology. The entire pattern was fabricated onto three different substrates: glass (Figure 3a, b), polyimide (Figure 3c), and paper (Figure 3d). Glass is a quite standard nonporous, flat, and rigid substrate, especially suited to test the performance of the printing technique under ideal conditions. The other two substrates are more technologically relevant, and should help to test the potential of CW-LIFT in areas with such a high impact as flexible plastic and paper electronics. The complete set of Ag electrodes (Figure 3a), around 2 cm × 2 cm in size, was printed in a single scan in only two seconds, a time short enough to overcome any potential solvent drying issues during printing. The CNF pad, on the other hand, is a square of 4 mm × 4 mm, which also took around two seconds to print; in spite that the pad is much smaller than the electrodes, up to 10 scans were required to uniformly cover its entire area, thus the longer time required to print it. The successful printing of the CNF pad is remarkable. First, CNFs are elongated structures with diameters below 100 nm but above 1 μm long. Such interesting aspect-ratio lays clearly beyond the limits of common inkjet nozzles. In this regard, Figure 3e illustrates very well the capability of CW-LIFT to print extremely different particle sizes: the large CNFs lay on a bed of tiny Ag nanoparticles in the interdigitated electrodes. Second, no complex formulation was required for the CNF ink (the CNF were simply suspended in water and the resulting liquid suspension was printed immediately thereafter) and therefore no chemical residual is expected once the solvent is evaporated after printing, an important issue for a gas sensor. This represents another significant advantage of LIFT in terms of simplicity, cost reduction and chemical purity: many materials can be printed without the need for complex ink formulations, a process which is usually time-consuming, complicated and expensive.1 It is also worth noting that both materials, Ag nanoparticles and CNFs, were printed with good definition not only on glass, but also onto the more challenging polyimide and paper substrates (Figure 3c, d). This is especially remarkable for paper, for which up to ten scans were required for the Ag electrodes due to its relatively high porosity (in the case of polyimide a single scan was enough). To determine the response of the sensors both to temperature and to the presence of gases, we tested the entire set following the procedures detailed in the Experimental Section of the Supporting Information. The surface mounted thermistor exhibits a negative temperature coefficient when exposed to a hot plate (Figure 3f). An ideal behavior is

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04409. Additional experimental details and calculation of the pool boiling hypothesis (PDF) Movie S1 showing the transfer dynamics (MP4)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 29416


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Pere Serra: 0000-0002-0676-1447 Author Contributions

The manuscript was written through equal contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by MINECO of the Spanish Government (Projects TEC2014-54544-C2-1-P and TEC2015-72425EXP) and also by Fondo Europeo de Desarrollo Regional ́ (FEDER). The authors thank E. Lendinez for his help in the acquisition of the time-resolved images corresponding to pulsed LIFT. A.C. acknowledges support from the 2015 edition of BBVA Foundation Grants for Researchers and Cultural Creators.

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Low-Cost Fabrication of Printed Electronics Devices through

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